1. Field of the Invention
The methods and compositions disclosed herein are broadly concerned with protecting plants or seeds from soil-borne fungal diseases. The composition may include a triterpenoid isolated from Glycyrrhiza glabra and a polymer. The triterpenoid may be Carbenoxolone disodium salt. The polymer coating decreases the miscibility of the Carbenoxolone disodium salt. The methods may include applying the composition to the plant's seeds, roots, tubers and/or foliage. The methods may also include applying the composition to the soil surround surrounding the plant. The composition may be applied as an aqueous solution or as dry particulates, and may be used for the treatment of soybean plants and seeds.
2. Prior Art
The soybean (Glycine max) is of worldwide significance both economically and agriculturally. Today, soybeans are grown in more than twenty-nine (29) states. Currently, the United States is the leading world producer of soybeans, accounting for nearly 2½ billion bushels per year. Animal feed accounts for nearly 75% of all domestic soybean meal consumption each year. Soybeans are also the foremost provider of protein and oil. A sixty (60) pound bushel of soybeans yields about forty-eight (48) pounds of protein-rich meal and eleven (11) pounds of oil. Products from soybeans include: tofu, soy milk, soy sauce, plywood, plastics, particle board, printing inks, soap, candy products, cosmetics and antibiotics, thus making soybeans a very economically important and diverse crop.
In 2005, the United States Department of Agriculture's National Agricultural Statistics Service (“NASS”) reported production of 3.09 billion bushels of soybeans, with an estimated record high average yield of 43.3 bushels per acre. Despite this encouraging yield data, disease is still problematic to soybean farmers and results in loss of yield. In 2004, the University of Missouri-Delta Center reported that disease resulted in up to 485,000,000 bushels lost, approximately 2.4 billion dollars, in the United States. Furthermore, the incidence of Macrophomina phaseolina accounted for the loss of 10,849,000 of those bushels, at approximately 54 million dollars, while Phytophthora rot accounted for the loss of 57,116,000 bushels, at approximately 285 million dollars.
Soil-borne fungal diseases represent some of the most difficult diseases to control and manage due to their persistence, virulence and longevity. Reliance upon chemically synthesized pesticides in agriculture has resulted in a number of social and ecological problems, and a number of these pesticides are under evaluation by Federal agencies to assess their safety and environmental impact. With the strong public interest and Federal mandates to develop safer pesticides for the environment, there is strong support in the development of seed treatments to help manage these plant diseases. An alternate approach to controlling plant diseases is the use of natural products, specifically plant-derived compounds.
Macrophomina phaseolina (“Charcoal rot”), Phytophthora spp. (“Phytophthora rot”), Pythium spp., Phoma spp. and Rhizoctonia solani are some of the most prevalent and destructive soil-borne diseases in the Midwest. Due to pathogen persistence in over-wintering periods, as well as predictable environmental stresses on the plants, repeated planting of soybeans in a field increases the risk of infection. Due to this complexity in host-pathogen interactions, disease is difficult to control.
Although there are many available varieties of soybeans, no such cultivar exists that is genetically or otherwise resistant to all of these soil-borne diseases. Furthermore, no single fungicide has been developed to effectively control these pathogens under all environmental conditions. Specific methods explored for controlling disease include commercial fungicides, crop rotation, and regulated irrigation, though none of these have proven to be completely effective.
The host range of Phytophthora spp. is a very diverse, with Phytophthora infestans being one of the most infamous of the genus and is the cause of potato late blight, the causal agent of the Irish Potato Famine which destroyed crops in many areas of Europe in the 1840's. Potato late blight has been classified as one of most devastating pathogen events in human history. Phytophthora sojae, the cause of Phytophthora root rot of soybeans, was first described over forty (40) years ago, but remains epidemic and a problem for soybean producers worldwide. Infections of Phytophthora spp. causes symptoms including pre- and post-emergence damping-off, stem and trunk cancer, root and tuber rot, wilting and leaf blight. Phytophthora rot can attack plants at any stage of development. In young plants, symptoms include yellowing, wilting and collapse/rot of the root and vascular system. In mature plants, symptoms vary and include reduced vigor, yellowing of the leaves, discoloration of the stem, and wilting and death.
Phytophthora disease incidence is most common is poorly drained, clay soils, but can occur in well-drained, lighter soils if saturated for an extended period of time. Development of root rot in well drained, saturated soils is usually coupled with immature, developing plants having little defense. Other factors that affect crop losses are cultivar susceptibility, rainfall, soil type and soil compaction. Drip irrigation, used to supplement natural rainfall, increases the likelihood of disease development in crops that require additional water.
Common methods for controlling Phytophthora rot include race specific cultivars, improved drainage and tillage methods, chemical control to protect young emergent plants, antifungal seed treatments and crop rotation. Race-specific resistance is effective against certain races of the pathogen, but not all species. Improved drainage and tillage efforts have also been explored for further control of the spread and severity of Phytophthora rot. In most cases, conventional tillage practices bury surface residues and loosen surface and soil layers. Reduced till and no-till practices leave crop residue on the soil surface for decomposition. Another method to reduce disease severity is the use of seed or foliar compounds that provide antifungal activity. Chemical antifungal agents available are BSR-101, Corsoy 79 and Ridomil, which have shown to reduce the severity of Phytophthora when used in combination.
Foliar applications of Zoxamide have been shown to protect against Phytophthora infestans. Combination antifungal treatments Zoxamide plus mancozed resulted in lower levels of tuber infection, compared to previous individual treatments with chlorthalonil (Bravo), azoxystrobin (Quadris™), or manozeb (Dithane™). Phosphite foliar sprays have also been used to prevent infection by wind driven zoospores with treated plants showing significantly lower leaf and petiole symptoms than untreated plants. In soybeans, methods for control include variety selection, metalaxyl (Ridomil™) (N-(2.6-dimethylphenyl)-N-(methoxyacetyl)-DL-alanine methyl ester), metalaxyl (Allegiance™, Gustafons), mefenoxam (Apron XL™, Syngenta) fungicide use and potash fertilizer. These compounds have been shown to reduce the severity of Phytophthora root rot when used in combination with variety selection or resistant cultivars. However, species of Phytophthora have shown resistance to metalaxyl, especially P. erythroseptica, P. citricola, P. nicotinanea and P. parasitica. 
Macrophomina phaseolina has been termed Charcoal rot due to the numerous black sclerotia produced during the pathogen's proliferate stage of growth in preparation for over-wintering. Charcoal rot ranks high among economically important pathogens for soybean disease, following soybean cyst nematode, Phytophthora rot, and seedling disease. M. phaseolina has been known to cause seedling blight, Charcoal rot and root rot in at least five hundred (500) different plant species.
Charcoal rot infection is soil-borne and/or seed-borne and remains dormant until seeds germinate. It has been shown that more than 60% of developing colonies originate from sclerotia; concurrently, seedling emergence can be reduced by as much as 60% from planting already infected seeds. The symptoms of Charcoal rot become apparent during times of midsummer, hot, dry weather. The ability of the pathogen to cause disease is based on many factors, including the potential of the inoculum to cause disease, soil moisture, temperature, seed quality, seed density and level of propagules present. In post-emergence disease development, damping-off or wilting is the most common symptom. Later season symptoms occur during reproduction, and include, smaller or dwarf leaflets, yellowing or wilting of leaflets due to early senescence, reddish or brown discoloration of the pith of the tap root and black streaks in crown tissue. The most common diagnostic symptoms are sloughing off of cortical tissue, silvery-gray speckled appearance of the infected tissue and wilting of the crown of the plant, known as a “Shepherd's crook.” All these symptoms are all due to the presence of sclerotia in the plant's vascular system. Seed yield is also reduced under these infection conditions.
Many cultural practices have been developed to manage the effects of Charcoal rot, and methods employed to reduce plant damage and increase overall plant health include decreasing seedling densities, planting later-maturity group cultivars, increasing soil fertility or crop rotation with less susceptible hosts. Additional methods investigated for improving plant health include biological control methods or hyperparasites, irrigation, fumigation of infested fields and the use of genetic resistant/tolerant cultivars.
Management strategies for controlling disease that include crop rotation and fall plowing to reduce inoculum densities in soil have been shown to be of limited benefit due to the wide host range of Charcoal rot. Soil populations of sclerotia seem to be reduced as soybeans are grown less frequently in crop rotation, even though other crops are susceptible. Furthermore, no fully resistant soybean germplasm exists, so improvement in disease resistant soybean cultivars has been limited. Many commercial and experimental plant varieties have been explored for resistance to M. phaseolina, and these cultivars do not show direct resistance to Charcoal rot infection, but they do seem to limit the growth rate of the fungus within the host. Additionally, lower plant populations or planting density have been explored to lessen the severity of Charcoal rot. This method is employed to promote rapid and strengthened growth and growth patterns, which proves beneficial in conjunction with mid-season irrigation and limited soil moisture. Water management and irrigation alleviate mid-season drought stress, and can limit, but not prevent colonization, of soybeans by sclerotia.
Tillage relationships with disease development have also been explored, but colonization of soybean roots occurred regardless of the tillage method used. Different soybean maturity groups and later planting dates have also been explored for disease control and resulted in later season flowering during times when the temperatures are lower and soil moisture is higher. However, varying planting dates can lead to problems with other pathogens, like later maturing soybean groups showing resistance to common pathogens, including the soybean cyst nematode, Heterodera glycines. Soil fumigation methods used to control pathogen populations in the soil prior to planting are of little benefit because the pathogen is found in both the seed and in the soil. Additional control methods explored include regulating such factors as soil moisture, temperature and nutrient levels to inhibit germination or growth of Charcoal rot.
With these disease management challenges, development of biological and natural control methods for M. phaseolina is an important avenue for exploration. By utilizing already present natural compounds and microbial parasites, there is less need for chemical fungicides and other synthetic chemical control measures. Bacterial parasites or sclerotial parasites have shown a reduction in germination of sclerotia. Natural plant-derived compounds have also been explored as alternatives for fungal disease management. Plant compounds such as lipids and lipid derivatives have also been shown to inhibit fungal growth in vitro, and in field trials, both as seed or foliar treatments.
General concern surrounds the uses of chemical pesticides in the United States because of possible environmental or human safety issues. Development of natural compounds for the control of disease and pests is of great interest. Natural plant derived compounds have been under investigation as possible control agents for some time.
Terpenoids and essential oils are secondary metabolites of plants. Carbenoxolone disodium salt is a terpenoid isolated from Licorice Root (Glycyrrhiza glabra) and a synthetic derivative from glycyrrhizinic acid. Carbenoxolone has anti-inflammatory properties and a noted binding affinity for albumin. Terpenoids disrupt the cell membrane's lipophilic compounds and are synthesized from acetate units and originate from the same compounds as fatty acids. Many uses of terpenoids have been described for the control of human diseases. Terpenoids from plants, like basil, have been shown to inhibit Salmonella, while others like betulinic acid have been shown to inhibit HIV.
Thus, there exists a need for a cost-effective, environmentally friendly composition and methods for effectively treating and/or preventing diseases in plants and seeds.